The present invention relates to methods of recording line elements such as lettering, frame lines, signets, edge spikes etc., in the exposure of film for the production of printing blocks or formes, and generally seeks to improve the methods existing today in the art.
Electronic appliances, so-called scanners, which perform color correction and screening i.e. the division of the images or of whole pages into picture elements as required for printing purposes, are applied in increasing degree to obtain the colour separation for multi-colour printing, apart from the conventional process. Letters, frame lines, signets, edge spikes, frames etc., are very frequently line images contained in such pictures or pages. If such line image elements were simply to be set up in screen form together with the continuous-tone pictures, the screen superimposition on particular contours would cause a stair-shaped distortion of the line contour which had been smooth in the original, which has an unacceptably disturbing effect in printed reproduction. The color separations are now commonly obtained in such manner that only the continuous-tone pictures are set up in screen form in electronic instruments or cameras. These half-tone color separations are then mounted together with a film which bears the non-screened line image. An intermediate negative of this "montage" is produced on a third film. This is copied again and provides a colour separation which then bears the combination of half-tone pictures and non-screened line images and is utilised for printing block production. It is thus mandatory to follow this procedure, which is costly in respect of equipment and time, which complementarily also sets up considerable uncertainty in the process as a whole because of the numerous copying processes, if it is wished to retain the smooth outlines of the line drawing in the half-tone pictures as far as the printing block.
It may well be possible to improve the line reproduction in the electronic instruments by scanning both data, namely continous-tone as well as line drawings, as well as recording these, with multiple line resolution: for example doubled or tripled. At a predetermined recording speed, the processing period would increase in step with the increase in resolution, which is equally unacceptable for practical operation. Moreover, a demand for increased storage arises upon storing whole images.
It is a specific object of the invention to avoid or minimise the drawbacks referred to and to establish a method for inserting line images into the half-tone color separations with improved reproduction of the contours, in scanners.